1. Field of the Invention
This invention relates in general to a drill for drilling a deep hole, and more particularly to such a drill having a long tool life without suffering from its breakage due to chip packing or poor chip removal in a deep-hole drilling operation at a high drilling speed.
2. Discussion of the Related Art
There is widely known a deep-hole drill having a chip evacuation flute or flutes whose axial length is at least five times or at least ten times as large as a diameter of the drill, for drilling a deep hole having a large depth that is at least five times or at least ten times as large as a diameter of the hole. As one example of the deep-hole drill, JP-U-H5-60715 (publication of unexamined Japanese Utility Application laid open in 1993) discloses a drill in which each chip evacuation flute has a width larger in an axially proximal end portion of the main body (i.e., a portion of the main body adjacent to the shank) than in an axially distal end portion of the main body, in the interest of preventing chip packing. As another example of the deep-hole drill, JP-A-H5-261612 (publication of unexamined Japanese Patent Application laid open in 1993) discloses a drill in which the web thickness is smaller in an axially proximal end portion of the main body than in an axially distal end portion of the main body. As still another example of the deep-hole drill, JP-A-S63-89210 (publication of unexamined Japanese Patent Application laid open in 1988) discloses a drill in which the surface of each chip evacuation flute is finished by a polishing operation and has a surface roughness Rz of not larger than 3 μm.
However, in a drilling operation with the drill disclosed by JP-A-H5-261612 in which the web thickness is changed in steps rather than continuously, smooth evacuation or removal of cutting chips from the hole is impeded due to the stepped change of the web thickness, suffering from a risk of the breakage. For example, where this conventional drill is adapted to have a diameter D of not larger than 8 mm for drilling a deep hole having a depth that is at least ten times as large as the diameter D of the drill, the drill is not capable of always drilling the deep hole in a high-speed drilling condition in which the drill is fed at a feed rate f of at least 0.10 mm/rev while being rotated at a peripheral velocity V of at least 80 m/min. In such a high-speed drilling condition, the drill sometimes suffers from its breakage due to chip packing or poor chip removal from the drilled hole. Therefore, the deep-hole drilling operation can not be satisfactorily achieved by the conventional drill, without reducing the feed rate f and/or the peripheral velocity V, or without adopting a so-called “step drilling” (i.e., feeding the drill in an intermittent manner).